Global Warming Forecast for Amazon Rain Forest: Dry and Dying

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The Amazon rain forest's dry season lasts three weeks longer than
it did 30 years ago, and the likely culprit is global warming, a
new study finds.

Rain falls year-round in the Amazon, but most of the annual
deluge drops during the wet season. (The rainy season's timing
varies with latitude.) Scientists think that a longer dry season
will stress trees, raising the risk of wildfires and forest
dieback. The
forest's annual fire season became longer as the dry season
lengthened, according to the study, published today (Oct. 21) in
the journal Proceedings of the National Academy of Sciences.

"The length of the dry season in the southern Amazon is the most
important climate condition controlling the rain forest," Rong
Fu, a climate scientist at The University of Texas at Austin's
Jackson School of Geosciences, said in a statement. "If the dry
season is too long, the rain forest will not survive."

The new findings forecast a more parched future for the Amazon
rain forest than the climate report released last month by the
Intergovernmental Panel on Climate Change (IPCC), the study
authors said. The IPCC models predict the Amazon dry season will
last three to 10 days longer by 2100.

But with the dry season already spanning an extra week each
decade since 1979, the Texas team said the future effects will be
more severe.

"The dry season over the southern Amazon is already marginal for
maintaining rain forest," Fu said. "At some point, if it becomes
too long, the rain forest will reach a tipping point."

Fu and her colleagues analyzed rainfall patterns across the
Southern Amazon rain forest since 1979, and plugged the data
into 50 simulations from eight climate models. The climate models
from the IPCC's AR5 report, released in September, reported
smaller dry season changes than actually measured since 1979.
This means the IPCC models likely underestimate future
predictions of rain forest climate change effects, the
researchers conclude.

Global
warming can limit tropical rainfall across the southern
Amazon in two ways, Fu explained. First, shifts in the southern
jet stream can block cold fronts that trigger rainfall. (In the
Northern Hemisphere, extremes in the northern jet stream pattern
have been linked to wacky weather, such as the unusually warm
winter in 2012.) Rising surface temperatures, attributed to
global warming, also make it harder for storms to start. The heat
inhibits "convective energy," keeping warm, dry air near the
surface from rising and mixing with cool, moist air above.